Path: utzoo!utgpu!attcan!uunet!husc6!bloom-beacon!apple!voder!kontron!optilink!cramer
From: cramer@optilink.UUCP (Clayton Cramer)
Newsgroups: comp.misc
Subject: Re: Mythical microprocessors
Message-ID: <331@optilink.UUCP>
Date: 4 Aug 88 16:17:15 GMT
References: <677@buengc.BU.EDU> <7650005@vx2.GBA.NYU.EDU> <17599@glacier.STANFORD.EDU>
Organization: Optilink Corporation, Petaluma, CA
Lines: 46

In article <17599@glacier.STANFORD.EDU>, jbn@glacier.STANFORD.EDU (John B. Nagle) writes:
> 
>       The Z80000 was supposed to be a follow-on to the Z8000, a modestly
> successful 16-bit micro very much like the PDP-11 in architecture.
> Unfortunately, the Z8000 had almost exactly the same limitations as
> the PDP-11 in terms of address space and organization of the memory
> management system.  The architecture simply would not let a single
> process use more than 64K code and 64K data.  This killed the product line.
> There were a number of UNIX boxes built using the Z8000, including some
> from Zilog, but it never really caught on.
> 
> 					John Nagle

The Z8000 consisted of two related processors: Z8002 and Z8001.  The Z8002
was limited to 64K code and 64K data space -- but there is a little more
to the story than that.

The Z8000 consisted of a total of six address spaces: System Code, System
Data, System Stack, Normal (application) Code, Normal Data, and Normal
Stack.  Each address space was 64K is size.  While the Z8002 was still
limited to 64K in each address space, it meant that your operating
system could take 64K of code, 64K of data, and then a 64K stack, with
no danger of any of them intruding on each other, and the application
programs had their 64K of code, of data, and stack to play in.  Not as
elegant as a flat address space, or even the 8086 segmentation scheme,
but as an alternative to a Z80, it was a giant improvement.

The Z8001 supported a very clumsy memory segmentation scheme that enabled
addressing a total of 128 separate 64K segments for each address space --
or 6 * 2^23 bits of address space.

The Z8000 family was much more successful in Europe than in America, but
there were applications for it, including (if memory serves me correctly)
the F16 fire control system.

The Z8000 had a very elegant instruction set (ignoring the memory 
addressing) for a microprocessor, in my opinion, much superior to the
68000.  It had 16 general registers, each of which could be used for
practically anything (major restriction being that 0 couldn't be used
for indexing), unlike the 68000's division into D and A registers.

The Z8000 was also very fast, when it came out in 1981-82.  If it had
appeared two years earlier, I think it likely that the 68000 would have
suffered greatly by comparision.

Clayton E. Cramer